Investigation of Campanian-Maastrichtian planktic foraminifera in north Tunisia reveals that the late Maastrichtian not only ends with a mass extinction, but also attains maximum species diversity during theirevolutionary history. Maximum species diversity is reached during global cooling in the early late Maastrichtian over a 600 kyr interval (69.1-69.7 Ma) when species richness nearly doubled with the evolution of many rugoglobigerinids and globotruncanids. No species extinctions occur at this time and there is little change in the relative abundanceof existing species, whereas new species did not evolve into numerically large populations during the succeeding late Maastrichtian. This suggests that species originations did not result in major competition and that the early-late Maastrichtian climatic cooling may have resulted in increased habitats and nutrient supply for marine plankton. The onset of the permanent decline in Cretaceous species richness began at65.9 Ma and accelerated during the last 50-100 kyr of the Maastrichtian, culminating in the mass extinction of all tropical and subtropicaltaxa at the end of the Maastrichtian. Climate changes appear to be responsible for both the rapid evolutionary activity in the early late Maastrichtian, as well as the gradual decline in species richness near the end of the Maastrichtian, although the additional stress imposed on the ecosystem by a bolide impact is the likely cause for the final demise of the tropical and subtropical fauna at the K-T boundary.